Abstract

Studies focused on the intrinsic molecular subtypes (Luminal A and B, Basal-like, HER2-enriched, Claudin-low, Normal Breast-like) are improving our understanding of the biologic heterogeneity of breast cancer. However, it is still not entirely clear if these distinct molecular entities are well represented in established cell lines derived from either breast cancers or normal mammary glands. In this study, we evaluated the relationships of a large in vitro panel of cell lines with (1) tumors and (2) four fluorescent activated cell (FAC) sorted fractions from the normal breast that are the stromal, mammary stem cells/bipotent progenitors (MaSC/BiPs), luminal progenitors (pL), and mature luminal cells (mL). First, by integrating genomic data of 337 human breast tissue samples with 65 breast cancer cell lines (BCCLs) and 22 human mammary fibroblasts and epithelial cells (HMFs and HMECs), we observed that all the molecular subtypes are identified in vitro, except for the Luminal A and Normal Breast-like tissue based subtypes. Interestingly, BCCLs showed higher expression of proliferation-related genes than their in vivo subtype counterparts. Secondly, using EpCAM and CD49f surface markers, we FAC sorted and characterized stromal, MaSC/BiPs, pL and mL from at least three independent reduction mammoplasties (as described in Lim et al. Nat Med 2009). Integration of genomic data, FACS (CD49f, EpCAM, CD44 and CD24) and immunofluorescent imaging (keratin 5, 8 and vimentin) revealed that BCCLs recapitulate the differentiation hierarchy observed in the normal mammary gland, with Claudin-low cells showing a mesenchymal phenotype (stromal-state), Basal-like cells showing a basal/luminal phenotype (pL-state), and Luminal B cells showing a luminal phenotype (mL-state), whereas HER2-enriched cells showed an intermediate pL-mL state. Interestingly, this approach allowed us to identify Basal-like (EpCAM+/CD49f+) and Claudin-low (EpCAM-/low/CD49f+) subpopulations of cells within a subset of triple-negative BCCLs (SUM149PT, HCC1143 and HCC38). These two subpopulations of cells showed distinct migratory capability and were not clearly identified using the CD24/CD44 cancer stem cell markers. Finally, we observed that HMECs show a unique mesenchymal and basal-like phenotype that resembles the MaSC/BiP-state, and can be modeled in vitro to study the stromal-Claudin-low transformation. The results presented here should help improve our understanding of the breast cell line model system through the appropriate pairing of cell lines with their relevant in vivo tumor and normal cell counterparts.